Oki B412dn User Manual page 233

There are fills at the top Insert a margin into the top edge of the paper.
edge of the paper.
An envelope is creased.
The paper is damp.
An envelope is printed under high
temperature and high humidity
conditions.
The envelope is creased even after
trying the above solutions.
Use paper stored under proper temperature and humidity
conditions.
Select [MENUS] > [PRINT ADJUST], and then set [HIGH HUM.
MODE] to [ON].
Load an envelope so that the flap faces the printer, and then turn
the orientation 180 degrees in the printing preferences on the
printer driver.
232
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Reference
About Paper Types That
Can be Loaded and
Storage Methods
MENUS
About Screens and
Functions of Printer
Drivers